I thus tried the GRUB2 tool that is included in KDE. Fortunately, in a matter of three clicks, KDE reinstalled the PicarOS GRUB2 to the distro's partition. After that, I ran GRUB customizer and it added the missing entry.

I cannot describe Eimi's happiness when the computer booted to PicarOS once again. She played a lot with GCompris and drew many pictures.

PicarOS is a child's magnet without any doubt. The comfort with which my little daughter uses the computer when booted to that OS is truly notorious...